Methods for extracting and retrieving the uranium present in an aqueous solution including phosphoric acid

ABSTRACT

The invention also relates to a method for retrieving uranium (VI) from an aqueous solution including phosphoric acid.

TECHNICAL FIELD

The present invention relates to the field concerning the extraction of uranium present in an aqueous medium containing phosphoric acid.

More particularly, it relates to a method for extracting uranium and more specifically uranium in oxidation state +VI, denoted uranium(VI) or U(VI), this uranium being present in an aqueous solution also comprising phosphoric acid.

The invention also relates to a method for recovering the uranium(VI) present in said aqueous solution.

The aqueous solution from which the uranium(VI) can be extracted, or from which it can be recovered, may notably be an aqueous solution resulting from the attack of a natural phosphate by sulfuric acid.

The present invention particularly finds application in the treatment of natural phosphates to recover the uranium value contained in these phosphates.

STATE OF THE PRIOR ART

Natural phosphates, also called phosphate ores, are used for the production of phosphoric acid and fertilizer. They contain uranium in amounts that can vary from a few tens of ppm to several thousand ppm, as well as variable amounts of other metals.

The potential recovery of the uranium contained in these natural phosphates is a few thousand tonnes per year, this representing a non-negligible source of uranium supply.

In methods currently used to recover this uranium contained in natural phosphates, these natural phosphates are subjected to attack by sulfuric acid. This attack converts tricalcium phosphate to phosphoric acid and results in solubilising the uranium together with various other metals, in particular iron which remains the majority metal impurity.

The actual recovery of uranium(VI) is therefore carried out from these concentrated aqueous phosphoric acid solutions that shall be called “aqueous phosphoric acid solutions” in the remainder of the present description.

At the present time, several routes are known for extracting the uranium contained in said aqueous phosphoric acid solutions.

A first route was particularly described in documents U.S. Pat. No. 3,711,591 and WO 2013/167516, respectively referenced [1] and [2] in the remainder of the present description and at the end hereof.

For this first route, the aqueous solution containing phosphoric acid and uranium is subjected to hydrometallurgical treatment based on liquid-liquid extraction, a technique whereby this aqueous solution, or aqueous phase, is placed in contact with an organic phase comprising one or more extractants to obtain extraction, in the organic phase, of the uranium contained in the aqueous phosphoric acid solution.

While the liquid-liquid extraction processes described in these documents [1] and [2] prove to be satisfactory, it nevertheless remains that they both have recourse to the use of organic solvents which generally have very low flash points or flammability points. Said organic solvents are therefore flammable and both the use and storage thereof can raise problems of industrial safety but also of environmental safety.

In addition, and conventionally, these liquid-liquid extraction processes require post-treatment of the extraction raffinate, corresponding to the aqueous phase from which the uranium is extracted, in order to remove residual traces of organic solvents therein, either further to a phenomenon of entrainment of these organic solvents, or on account of their partial solubility in an aqueous phase. This post-treatment step allows the directing of this extraction raffinate into industrial circuits.

To overcome these disadvantages generated by the use of organic solvents, a second route to extract uranium has been proposed.

This second route uses solid-liquid extraction, whereby uranium is extracted from an aqueous phosphoric acid solution by contacting this aqueous solution with a water-insoluble material comprising functional chemical groups capable of retaining the uranium either by ion exchange or by chelation.

Among the proposed materials recognized as allowing the extraction of uranium from aqueous phosphoric acid solutions, particular mention can be made of the organic materials such as taught in documents U.S. Pat. No. 4,599,221 and U.S. Pat. No. 4,402,917, respectively referenced [3] and [4].

Document [3] proposes an organic material marketed under the trade name Duolite™ ES-467, having functional groups meeting formula —CH₂NH—CH₂—PO₃ ⁻² attached to a macroporous polystyrene matrix.

Document [4] proposes extracting uranium from an aqueous solution comprising between 5 mol/L and 8 mol/L of phosphoric acid, by means of a solid polymeric substrate also called a “resin”. This solid polymeric substrate is composed of polyacrylates or styrene/vinylbenzene copolymers and is impregnated with an organophosphorus extractant. Several resins marketed under the trade name Amberlite™ (XAD-4, XAD-7, XAD-8 or XE-299) were used as solid polymeric substrate.

However, in these documents [3] and [4], the extraction processes require that the uranium present in oxidation state +VI in the aqueous phosphoric acid solutions resulting from sulfuric attack of natural phosphates, should be previously reduced to oxidation state +IV before it is possible to carry out actual extraction of the uranium.

Document WO 2014/018422, referenced [5], also proposes a method for extracting the uranium contained in an aqueous phosphoric acid solution by implementing one or two cycles to contact said aqueous solution with an ion exchange resin, with continuous operation. Among the resins described in this document [5], the commercial resins Lewatit™ TP260, Amberlite™ IRC-747 or Purolite™ S-930 are cited.

It is to be noted that the method described in document [5] is not illustrated by any example of extraction allowing an insight into the performance of the associated method both regarding the extraction yield obtained and the selectivity towards iron.

It is therefore the objective of the invention to propose a method for extracting uranium(VI) from an aqueous phosphoric acid solution that is particularly efficient irrespective of the concentration of phosphoric acid in this aqueous solution. In particular, it must be possible to implement this method to extract uranium(VI) from so-called “concentrated” aqueous phosphoric acid solutions, such as aqueous solutions resulting from attack of a natural phosphate by sulfuric acid having a typical phosphoric acid concentration of at least 5 mol/L.

The method of the invention must also allow highly selective extraction of uranium(VI) over other metal cations likely to be contained in the aqueous phosphoric acid solution and, in particular, over iron(III).

The method of the invention for extracting uranium(VI) must generally overcome the shortcomings of the prior art, and in particular must be free of the above-mentioned disadvantages, both for liquid-liquid extraction and for solid-liquid extraction.

More particularly, it must be possible for the method of the invention to be implemented under optimised conditions of industrial safety and environmental safety, circumventing the use of organic solvents such as described in documents [1] and [2] and, in particular, circumventing implementation of a post-treatment step to remove traces of residual organic solvents contained in the aqueous phosphoric acid solution from which the uranium is extracted, as is the case with the liquid-liquid extraction methods in these documents [1] and [2].

This method must therefore, and in addition, involve a reduced number of steps compared with prior art methods. For example, the method of the invention must not have recourse either to a reduction step of uranium(VI) to uranium(IV) prior to extraction properly so-called, but it must allow direct extraction of this uranium contained in oxidation state +VI in aqueous phosphoric acid solutions such as those resulting from sulfuric attack of natural phosphates.

DESCRIPTION OF THE INVENTION

These objectives set forth above and others are reached first with a method for extracting uranium(VI) from an aqueous solution S containing phosphoric acid, this method comprising the placing of the aqueous solution S in contact with an organic material, followed by separation of the aqueous solution and the organic material.

According to the invention, the organic material comprises a solid polymeric substrate impregnated with a compound meeting the following general formula (I):

where:

-   -   m is an integer of 0, 1 or 2;     -   R¹ and R², the same or different, are a linear or branched,         saturated or unsaturated hydrocarbon group having 6 to 12 carbon         atoms;     -   R³ is:         -   a hydrogen atom;         -   a linear or branched, saturated or unsaturated hydrocarbon             group having 1 to 12 carbon atoms and optionally one or more             heteroatoms;         -   a saturated or unsaturated hydrocarbon group comprising one             or more rings of 3 to 8 carbon atoms, the ring(s) optionally             comprising one or more heteroatoms; or         -   an aryl group comprising one or more rings, the ring(s)             optionally comprising one or more heteroatoms;     -   or else R² and R³ together form a group —(CH₂)_(n)— where n is         an integer ranging from 1 to 4;     -   R⁴ is:         -   a linear or branched, saturated or unsaturated hydrocarbon             group having 2 to 8 carbon atoms;         -   a saturated or unsaturated hydrocarbon group comprising one             or more rings, the ring(s) optionally comprising one or more             heteroatoms; or         -   an aromatic group comprising one or more rings, the ring(s)             optionally comprising one or more heteroatoms; and     -   R⁵ is a hydrogen atom or a linear or branched, saturated or         unsaturated hydrocarbon group having 1 to 12 carbon atoms.

The method of the invention therefore consists of solid-liquid extraction and therefore does not have the disadvantages related to the use of organic solvents as it is the case with liquid-liquid extraction.

The inventors have unexpectedly and surprisingly ascertained that an organic material comprising a solid polymeric substrate impregnated with a compound meeting general formula (I) such as defined above, allows uranium(VI) to be extracted from an aqueous phosphoric acid solution with high performance and high selectivity irrespective of the concentration of phosphoric acid in this aqueous solution. More particularly, this extraction is conducted by complexing this uranium(VI) by the compound meeting general formula (I), this compound itself being adsorbed on the solid polymeric substrate of the organic material.

This finding is all the more surprising since it goes against the teaching of document WO 2014/127860, recently published and referenced [6].

This document [6] proposes the use of a hybrid organic-inorganic material to extract uranium from aqueous phosphoric acid solutions. This organic-inorganic hybrid material comprises a solid substrate of inorganic type on which are covalently bonded molecules of organic type and which comprise an amidophosphonate unit capable of complexing and retaining uranium(VI) via this complexing mechanism. This solid substrate of inorganic type is presented by document [6] as being chemically more stable than the organic substrates of organic materials, such as those proposed by document [3].

Yet, as shown in the examples below, extractions conducted with the particular organic material used in the extraction method of the invention give very high performance. In particular, for a same molar concentration of phosphoric acid in the aqueous solution, the performance of the extraction method of the invention is higher than that of the extraction method in document [6].

In addition, the method of the invention allows direct extraction of the uranium contained in the aqueous acid solution, at its oxidation state +VI. Therefore, the method of the invention does not comprise a step to reduce uranium(VI) to uranium(IV) prior to the actual extraction step, contrary to the methods described in documents [3] and [4].

The organic material used in the method of the invention comprises a solid polymeric substrate impregnated with a compound meeting general formula (I) given above.

This compound, a bifunctional compound of amidophosphonate type, and the synthesis method thereof are known and described in document [2]. Readers are therefore prompted to refer to this document [2] for any details relating to this compound and the synthesis method thereof and, in particular, the definition given therein of expressions such as “linear or branched, saturated or unsaturated hydrocarbon group having x to y atoms”, “heteroatom”, “saturated or unsaturated, cyclic hydrocarbon group having x to y carbon atoms” or “aromatic group”.

In one particular version of the invention, the compound of the organic material meets general formula (I) above where m=0, whilst R¹, R², R³, R⁴ and R⁵ are such as previously defined.

In one advantageous version of the invention, the compound of the organic material meets the following particular formula (I-a):

where:

-   -   R¹ and R² are an alkyl group having 8 to 10 carbon atoms;     -   one from among R³ and R⁵ is a hydrogen atom and the other from         among R³ and R⁵ is an alkyl group having 4 to 10 carbon atoms;         and     -   R⁴ is an alkyl group having 4 to 6 carbon atoms.

In one particular version of the invention, the compound of the organic material is selected from among:

-   -   ethyl 1-(N,N-diethylhexylcarbamoyl)ethylphosphonate, denoted         DEHCEPE, meeting the particular formula (I-a) above where R¹ and         R² are both a 2-ethylhexyl group, R⁴ is an ethyl group, one from         among R³ and R⁵ is a hydrogen atom whilst the other from among         R³ and R⁵ is a methyl group;     -   ethyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, denoted         DEHCNPE, meeting the particular formula (I-a) above where R¹ and         R² are both a 2-ethylhexyl group, R⁴ is an ethyl group, one from         among R³ and R⁵ is a hydrogen atom whilst the other from among         R³ and R⁵ is an n-octyl group;     -   butyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, denoted         DEHCNPB, meeting the particular formula (I-a) above where R¹ and         R² are both a 2-ethylhexyl group, R⁴ is an n-butyl group, one         from among R³ and R⁵ is a hydrogen atom whilst the other from         among R³ and R⁵ is an n-octyl group;     -   butyl 1-(N,N-dioctylcarbamoyl)nonylphosphonate, denoted DOCNPB,         meeting the particular formula (I-a) above where R¹ and R² are         both an n-octyl group, R⁴ is an n-butyl group, one from among R³         and R⁵ is a hydrogen atom whilst the other from among R³ and R⁵         is an n-octyl group; and,     -   isopropyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, denoted         DEHCNPIP, meeting the particular formula (I-a) above where R¹         and R² are both a 2-ethylhexyl group, R⁴ is an isopropyl group,         one from among R³ and R⁵ is a hydrogen atom whilst the other         from among R³ et R⁵ is an n-octyl group.

In one more particular version of the invention, and particularly in the following examples, the compound impregnated in the organic material is butyl 1-(N,N-didthylhexylcarbamoyl)nonylphosphonate, denoted DEHCNPB.

The solid polymeric substrate of the organic material is more preferably formed of a polymer that does not or practically does not comprise functional groups likely to react with the metal cations contained in the aqueous solution S.

This solid polymeric substrate can be formed of a polymer comprising at least one repeat unit selected from among an olefin unit, benzene unit, acrylic ester unit and mixtures of these units.

In one advantageous variant, this polymer may be a divinylbenzene/styrene copolymer or an acrylic ester polymer.

In another advantageous variant, the solid polymeric substrate has a specific surface area of between 300 m²/g and 1000 m²/g (determined by the BET method).

In another advantageous variant, the solid polymeric substrate has a pore diameter of between 50 Å and 950 Å.

In another advantageous variant, the solid polymeric substrate is in the form of beads or pearls having a mean size exhibited by at least 90% of the beads by number, denoted d₉₀, is advantageously between 200 μm and 900 μm.

Solid polymeric substrates suitable for the organic material are notably those available from the Dow Chemical Company under the trade names Amberlite™ XAD-4, Amberlite™ XAD-7, Amberlite™ XAD-16 or Amberlite™ XAD-1180, Amberlite™ XAD-7 corresponding to a solid polymeric substrate containing an acrylic ester polymer, the other commercial references being solid polymeric substrates containing divinylbenzene/styrene copolymers. Other solid polymeric substrates containing divinylbenzene/styrene copolymers are also available from the Purolite Company under the trade names Hypersol-Macronet™ MN202 and Hypersol-Macronet™ MN500.

As previously indicated, the organic material is formed of the solid polymeric substrate in which the compound meeting general formula (I) is impregnated, this impregnation being characterized by adsorption of this compound on this solid polymeric substrate.

The impregnation of this compound in the solid polymeric substrate is conventionally obtained via wet process, i.e. by dissolution of the compound in a suitable volatile organic solvent followed by contacting of the solution obtained with the solid polymeric substrate so that this substrate becomes impregnated with the compound. After impregnation of the compound in the solid polymeric substrate, the excess of organic solvent is evaporated, preferably in vacuo, after which the organic material obtained is in dry form. This impregnation of the compound in the solid polymeric substrate via wet process allows an organic material to be prepared in which the bleed-out limit of the compound is not reached.

In one variant of the invention, the organic material comprises a weight percentage of at least 2.5 wt. % of this compound, impregnated in the solid polymeric substrate, relative to the total weight of the organic material.

However, a weight percentage of less than 2.5 wt. % of this compound relative to the total weight of the organic material is not excluded.

In one advantageous variant of the invention, the organic material comprises a weight percentage ranging from 10 wt. % to 70 wt. % and, preferably, from 20 wt. % to 60 wt. %.

The method for extracting uranium(VI) from an aqueous solution S comprising phosphoric acid, comprises:

-   -   placing the aqueous solution S in contact with the organic         material just described; then     -   separating the organic material containing uranium(VI) from the         aqueous solution from which the uranium has been extracted, and         which is said to be “uranium-depleted”.

At the contacting step of the aqueous solution S with the organic material, uranium (VI) is extracted by the organic material. This extraction more particularly takes place via complexing of uranium(VI) by the compound of general formula (I). At this contacting step, the concentration of uranium(VI) in the aqueous solution is reduced accordingly.

As shown in the examples below, it is observed that the method of the invention allows the extraction, with high extraction yield, of the uranium (VI) initially contained in the aqueous solution S, with an organic material having a loading capacity of uranium(VI) that is higher than that described in document [6].

In addition, when the aqueous solution S contains iron(III), it is observed that this iron(III) is extracted by the organic material. As for uranium(VI), this extraction takes place via complexing of iron(III) by the compound of general formula (I). It is to be noted that the amount of iron(III) adsorbed on the organic material remains very low.

This contacting of this aqueous solution S with the organic material can be obtained simply by mixing, for example using a system under agitation for sufficient time to allow the uranium(VI) to be extracted by the organic material.

In one advantageous variant of the extraction method of the invention, this contacting of the aqueous solution S with the solid organic material comprises at least one circulation of a mobile phase formed by the aqueous solution S on a stationary phase formed by the organic material.

This stationary phase formed by the organic material can be mobile and be in the form of a fluidised bed.

This stationary phase formed by the organic material can also be static and be arranged in a column.

In this second variant, the mobile phase formed by the aqueous solution S can be injected into the column containing the organic material. After circulation of this mobile phase on the stationary phase, a mobile phase depleted of uranium(VI) is collected at the outlet of the column.

This mobile phase depleted of uranium(VI) collected at the outlet of the column can be again injected into the same column for a second extraction, to recover the uranium(VI) that may not have been extracted during the first injection. The concentration of uranium(VI) in the liquid phase decreases with successive circulations of the mobile phase on the stationary phase.

The contacting of the stationary phase with successive circulations of aqueous solution collected at the outlet of the column allows optimised extraction of uranium(VI), even in the presence of iron(III) if any is contained in the aqueous solution S. Indeed, the amount of extracted uranium(VI), hence present on the stationary phase formed by the organic material, increases with each successive circulation of aqueous solution.

The separation of the aqueous solution from the organic material containing uranium(VI) and possibly iron (III) can be conducted using any usual separation technique for separating a solid from a liquid, such as filtration, centrifugation, . . . .

In the particular case in which the contacting step is formed of at least one circulation in the column of the liquid phase on the stationary phase, separation is conducted concomitantly with this contacting step.

Irrespective of the chosen contacting step, simple mixing or circulation in a column and/or of the separation step, the method for extracting uranium(VI) according to the invention is particularly simple to implement.

The method for extracting uranium(VI) from the aqueous solution S being completed, it is subsequently possible to strip the uranium(VI) from the organic material to recover this uranium(VI) for reuse thereof.

A further subject of the invention is therefore a method for recovering uranium(VI) from an aqueous solution S comprising phosphoric acid.

According to the invention, this recovery method comprises:

-   -   (a) extracting uranium(VI) from the aqueous solution S using an         extraction method such as described in the foregoing; and     -   (b) stripping the uranium(VI) from the organic material obtained         at step (a) by contacting the organic material obtained after         step (a) with a basic aqueous solution followed by separation of         the organic material and the basic aqueous solution, after which         uranium(VI) is recovered in the basic aqueous solution.

In other words, this method to recover uranium(VI) comprises the following steps:

-   -   (a) placing the aqueous solution S in contact with an organic         material comprising a solid polymeric substrate impregnated with         a compound meeting following general formula (I):

-   -   -   where:             -   m is an integer of 0, 1 or 2;             -   R¹ and R², the same or different, are a linear or                 branched, saturated or unsaturated hydrocarbon group                 having 6 to 12 carbon atoms;             -   R³ is:                 -   a hydrogen atom;                 -   a linear or branched, saturated or unsaturated                     hydrocarbon group having 1 to 12 carbon atoms and                     optionally one or more heteroatoms;                 -   a saturated or unsaturated hydrocarbon group                     comprising one or more rings of 3 to 8 carbon atoms,                     the ring(s) optionally comprising one or more                     heteroatoms; or                 -   an aryl group comprising one or more rings, the                     ring(s) optionally comprising one or more                     heteroatoms;             -   or else R² and R³ together form a group —(CH₂)_(n)—                 where n is an integer ranging from 1 to 4;             -   R⁴ is:                 -   a linear or branched, saturated or unsaturated                     hydrocarbon group having 2 to 8 carbon atoms;                 -   a saturated or unsaturated hydrocarbon group                     comprising one or more rings, the ring(s) optionally                     comprising one or more heteroatoms; or                 -   an aromatic group comprising one or more rings, the                     ring(s) optionally comprising one or more                     heteroatoms; and             -   R⁵ is a hydrogen atom or a linear or branched, saturated                 or unsaturated hydrocarbon group having 1 to 12 carbon                 atoms,         -   followed by separation of the aqueous solution and the             organic material; and

    -   (b) placing the organic material obtained after step (a) in         contact with a basic aqueous solution, followed by separation of         the organic material and the basic aqueous solution.

The Inventors have also evidenced the fact that step (b) of the recovery method according to the invention performs particularly well and allows quantitative recovery of the uranium(VI) previously extracted by the organic material.

In particular, and as shown in the examples below, the method of the invention allows the recovery of uranium(VI) with a yield of at least 95%, even much higher than 95%.

It is also noted that the method of the invention additionally allows particularly selective recovery of uranium(VI), in particular over iron(III), if any is initially contained in the aqueous solution S. At step (b), only uranium(VI) is stripped. Indeed, when in contact with the basic solution, iron(III) is precipitated in the form of iron hydroxide Fe(OH)₃ on the organic material and is therefore not stripped. It is to be noted however that the presence of this iron hydroxide precipitate on the organic material has no impact on the circulation of the different feed solutions in the column of organic material, the amount of iron(III) extracted at step (a) remaining truly minimum.

In this recovery method of the invention, step (a) is conducted using the extraction method defined above, it being specified that the advantageous characteristics of this extraction method, such as those relating to the compound meeting general formula (I) and/or to the solid polymeric substrate, can be taken alone or in combination.

At step (b) of the recovery method of the invention, the uranium(VI) extracted by the organic material is stripped by contacting this organic material with a basic aqueous phase.

After this contacting, the uranium(VI) stripped from the organic material is recovered in the basic aqueous solution.

The contacting of the organic material with the basic aqueous solution can be carried out by simple mixing, e.g. using a system under agitation, for sufficient time to allow the uranium(VI) to be stripped into the basic aqueous phase.

In one advantageous variant of the recovery method of the invention, this contacting of the organic material obtained after step (a) with the basic aqueous solution comprises at least one elution, with a mobile phase formed by the basic aqueous solution, of a stationary phase formed by the organic material.

As previously described for the extraction method, this stationary phase formed by the organic material can be mobile and in the form of a fluidised bed. It may also be static and be arranged in a column.

This advantageous variant for contacting the organic material with the basic aqueous solution may particularly be carried out in a column.

The separation of the organic material from the basic aqueous solution containing the uranium(VI) can be performed using any usual separation technique to separate a solid from a liquid, such as filtration, centrifugation, . . . .

In the particular case in which the contacting step consists of elution, in a column, of the stationary phase by the liquid phase, separation is conducted concomitantly with this contacting step.

Irrespective of the chosen contacting step, simple mixing or elution in a column, the implementation of the uranium(VI) recovery method of the invention is therefore particularly simple.

In one variant of the invention, the basic aqueous solution has a pH of 8 or higher.

The pH of the basic aqueous solution is advantageously between 8.5 and 12, preferably between 9 and 11.

In another variant of the invention, the basic aqueous solution is an aqueous solution of an alkaline metal salt or of an ammonium salt.

The alkaline metal or ammonium salt is advantageously a carbonate.

The alkaline metal can advantageously be selected from among sodium and potassium.

Said salt can be selected in particular from among sodium carbonate and ammonium carbonate.

In one advantageous variant, the recovery method of the invention may comprise, between steps (a) and (b):

(a′) washing with water of the organic material obtained at step (a).

This washing with water allows removal of residual traces of phosphoric acid present on the organic material.

The recovery method of the invention may also comprise regeneration of the organic material so that it is possible to envisage at least one subsequent extraction and, optionally, at least one subsequent recovery of uranium(VI) contained in an aqueous solution S.

According to the invention, the recovery method together with said regeneration of the organic material comprises, after step (b):

-   -   (c) placing the organic material obtained after step (b) in         contact with an acid aqueous solution, followed by separation of         the organic material and the acid aqueous solution, after which         the organic material is regenerated.

In one advantageous variant, the recovery method of the invention may comprise, between steps (b) and (c):

(b′) washing with water of the organic material separated at step (b).

This washing with water allows removal of residual traces of basic aqueous solution used at step (b) and which are present on the organic material.

At this step (c), the contacting of the organic material with an acid aqueous solution allows regeneration of the organic material via protonation.

Additionally, if iron(III) is precipitated in the form of iron hydroxide Fe(OH)₃ on the organic material, the contacting of this organic material with the acid aqueous solution causes dissolution of this precipitate and recovery of the iron(III) in the acid aqueous solution. Therefore, not only is the organic material regenerated via protonation but it is also freed of the poisonous iron hydroxide precipitate Fe(OH)₃.

The method of the invention therefore allows the selective and sequential recovery of uranium(VI) by implementing steps (a) and (b), then of iron(III) by implementing step (c).

This contacting of the organic material with the acid aqueous solution can be conducted by simple mixing, e.g. using a system under agitation for sufficient time to allow regeneration of the organic material and, optionally, dissolution of the iron hydroxide.

In one advantageous variant of the recovery method of the invention, this placing of the organic material separated at step (b)—and optionally washed at step (b′)—in contact with the acid aqueous solution comprises at least one washing with a mobile phase formed by the acid aqueous solution on a stationary phase formed by the organic material.

This advantageous variant for contacting the organic material with this acid aqueous solution may in particular be performed in a column.

The separation of the organic material from the acid aqueous solution containing iron(III) can be conducted using any usual separation technique to separate a solid from a liquid, such as filtration, centrifugation, . . . .

In the particular case in which the contacting step comprises at least one washing, in a column, of the stationary phase by the liquid phase, separation is carried out concomitantly with this contacting step.

Irrespective of the chosen contacting step, simple mixing or elution in a column, the implementation of the uranium(VI) recovery method with regeneration of the organic material according to the invention is therefore also particularly simple.

In one variant of the invention, the acid aqueous solution has a concentration of H⁺ ions of 20 mol/L or less.

This concentration of H⁺ ions in the acid aqueous solution is advantageously between 0.1 mol/L and 10 mol/L and, preferably, between 1 mol/L and 7 mol/L.

In another variant of the invention, the acid aqueous solution is an aqueous solution comprising at least one inorganic acid.

Said inorganic acid can be selected from the group formed by sulfuric acid, nitric acid, phosphoric acid and hydrochloric acid.

In one advantageous variant, this inorganic acid is sulfuric acid.

The aqueous solution S from which uranium (VI) can be extracted in accordance with the extraction method of the invention, or which is used at step (a) of the recovery method of the invention, may comprise phosphoric acid over a very broad range of molar concentrations, said molar concentration being at least 0.1 mol/L of phosphoric acid for example.

In one advantageous version, the aqueous solution S may comprise from 1 mol/L to 10 mol/L, preferably from 2 mol/L to 9 mol/L and, more preferably, from 3 mol/L to 7 mol/L of phosphoric acid.

As shown in the examples below, it is observed that the method of the invention allows the extraction, with high extraction yield, of the uranium(VI) initially contained in the aqueous solution S comprising 5 mol/L of phosphoric acid, without degradation of the organic material or of its solid polymeric substrate, contrary to the teaching of document [6].

The aqueous solution S may particularly be a solution resulting from attack of a natural phosphate by sulfuric acid.

In one advantageous variant of each of the extraction and recovery methods of the invention, the aqueous solution S has a redox potential of between 450 mV and 550 mV in relation to a Ag/AgCl reference electrode. Indeed, at such redox values, the uranium and iron contained in the aqueous solution S are in the form of uranium(VI) and iron(III).

Other characteristics and advantages of the invention will become apparent on reading the following description given with reference to appended FIGS. 1 to 5 and relating to an example of preparation of an organic material and to examples of extraction, illustrating the performance of said organic material used in methods to extract and recover uranium(VI) initially contained in different aqueous solutions comprising phosphoric acid.

Evidently these examples are solely given to illustrate the subject of the invention and do not in any manner limit this subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 gives curves reflecting first the changes in the concentration of uranium(VI) contained in the collected fractions of aqueous solution, denoted [U]_(S5) and expressed in mg/L, as a function of the number of resin bed volumes (denoted BV), and secondly the trend in the uranium(VI) loading capacity of the impregnated resin, denoted C_(U(S5)) and expressed in g of uranium(VI)/L, as a function of the number of resin bed volumes (denoted BV), after injection of a synthetic aqueous phosphoric acid solution S5 into a column comprising an impregnated resin conforming to the invention.

FIG. 2 reproduces the curves of FIG. 1 and gives the curves reflecting first the changes in concentration of uranium(VI) contained in the collected fractions of aqueous solution, denoted [U]_(S5) and expressed in mg/L, as a function of the number of resin bed volumes (denoted BV), and secondly the trend in uranium(VI) loading capacity of the impregnated resin, denoted C_(U(S6)) and expressed in g of uranium(VI)/L, as a function of the number of resin bed volumes (denoted BV), after injection of a synthetic aqueous phosphoric acid solution S6 into a column comprising an impregnated resin conforming to the invention.

FIG. 3 gives the curves reflecting the trend in uranium(VI) loading capacities, respectively denoted C_(U(Ri)), C_(U(R1)) and C_(U(R2)) and expressed in g of uranium(VI)/L, as a function of the number of resin bed volumes (denoted BV), after injection of the synthetic aqueous phosphoric acid solution S7 into three separate columns respectively comprising an impregnated resin conforming to the invention Ri, a resin R1 and a resin R2.

FIG. 4 gives the curves reflecting the changes in concentrations of uranium(VI) contained in the collected fractions of aqueous solution, respectively denoted [U]_(Ri), [U]_(R1) and [U]_(R2) and expressed in mg/L, as a function of the number of resin bed volumes (denoted BV), after injection of the industrial aqueous phosphoric acid solution SI into three separate columns respectively comprising an impregnated resin conforming to the invention Ri, a resin R1 and a resin R2.

FIG. 5 gives the curves reflecting the trend in uranium(VI) loading capacities, respectively denoted C_(U(Ri)), C_(U(R1)) and C_(U(R2)) and expressed in g of uranium(VI)/L, as a function of the number of resin bed volumes (denoted BV), after injection of the industrial aqueous phosphoric acid solution SI, into three separate columns respectively comprising an impregnated resin conforming to the invention Ri, a resin R1 and a resin R2.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS Example 1: Preparation of a Material According to the Invention

The preparation of the organic material, comprising a solid polymeric substrate and a compound meeting formula (I) detailed above, was carried out starting with the following compounds:

-   -   the solid polymeric substrate, hereafter called “resin”:         Amberlite™ XAD-7 trade mark resin marketed by the Dow Chemical         Company. This resin, formed of an acrylic ester polymer, is in         the form of beads having a particle size of between 250 μm and         840 μm, a specific surface area of 450 m²/g (BET method) and a         pore diameter of 300 Å; and     -   the compound: butyl         1-(N,N-diethylhexylcarbamoyl)-nonylphosphonate, denoted DEHCNPB.         This compound was synthesized in accordance with the teaching of         document [2], by implementing steps A, B, C and E of the         reaction scheme shown in FIG. 1 of this document [2].

Impregnation of the solid polymeric substrate with DEHCNPB was obtained by wet process.

For this process, the DEHCNPB was first dissolved in a volatile solvent. The mixture obtained was then placed in contact with the Amberlite™ XAD-7 resin for impregnation thereof with DEHCNPB.

The solvent was then evaporated in vacuo so that the resulting organic material was in the form of dry beads of impregnated resin, the weight proportion of DEHCNPB in the impregnated resin, or organic material, being 50 wt. %.

In the remainder of the present description, for ease of authoring, the expressions “organic material” and “impregnated resin”, which have the same meaning, are used.

Example 2: Properties of an Organic Material of the Invention to Extract and to Recover Uranium(VI) from Synthetic Aqueous Phosphoric Acid Solutions

2.1 Evaluation of U(VI) Extraction Capacity by Contact Tests

The capacity of the organic material in Example 1 to extract uranium(VI) from aqueous phosphoric acid solutions was determined by contact tests conducted in accordance with the following protocol:

-   -   250 mg of impregnated resin such as obtained in Example 1 were         mixed with 10 mL of an aqueous phosphoric acid solution formed         by a synthetic aqueous phosphoric acid solution comprising 1         mol/L of phosphoric acid and varying concentrations of         uranium(VI), and optionally iron(III); the redox potential of         this synthetic aqueous phosphoric acid solution was 550 mV (in         relation to the Ag/AgCl reference electrode);     -   the mixture obtained was left under agitation for 23 h, at         ambient temperature, using an incubator agitator; after which         the solid and liquid phases forming this mixture were separated         by filtration.

The concentrations of uranium(VI), determined by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), were measured:

-   -   in the initial synthetic aqueous phosphoric acid solution,         before contacting thereof with the organic material formed by         the dry beads of impregnated resin; and     -   in the liquid phase after contacting with the resin, or         filtrate.

The difference found between the concentration of uranium(VI) in the initial synthetic solution and in the filtrate corresponds to the amount of uranium(VI) extracted by the organic material.

The uranium(VI) loading capacity of this impregnated resin corresponds to the amount of uranium(VI) extracted by this impregnated resin. This loading capacity, denoted C_(U) and expressed in mg of uranium/g of impregnated resin (denoted mg U/g), was determined with the following formula:

$C_{U} = {\left( {C_{ini} - C_{fin}} \right) \times \frac{V_{solution}}{m_{resin}}}$

where:

-   -   C_(ini): concentration of uranium(VI) in the initial synthetic         aqueous solution of phosphoric acid (in mg/L)     -   C_(end): concentration of uranium(VI) in the filtrate (in mg/L)     -   V_(solution): volume of initial synthetic aqueous solution of         phosphoric acid contacted with the impregnated resin for 23 h         (in L)     -   m_(resin): weight of impregnated resin (in g)

The coefficient of distribution for uranium, denoted K_(d), was determined with the following formula:

$K_{d} = \frac{C_{U}}{C_{end}}$

where C_(U) and C_(end) have the same meaning as previously.

Table 1 below gives the results obtained with four initial synthetic aqueous solutions of phosphoric acid, denoted S1 to S4, having varying initial concentrations of uranium(VI) and optionally of iron(III).

TABLE 1 C_(ini) of Fe(III) C_(ini) of U(VI) C_(end) of U(VI) C_(U) Solution (g/L) (mg/L) (mg/L) (mg U/g) K_(d) S1 — 245 1 9.7 9.7 S2 — 493 6 19.0 2.8 S3 3 225 5 8.6 1.7 S4 3 477 21 18.6 0.9

The results in Table 1 show that, for a same ratio of liquid phase to solid phase, denoted Liquid/Solid below, the impregnated resin allows the extraction of 9.7 g and 19.0 g of uranium(VI) per kg of impregnated resin from solutions S1 and S2 only comprising uranium(VI) in respective initial concentrations in the order of 250 mg/L and 500 mg/L.

It is to be noted that such results are much higher than those observed with a hybrid organic-inorganic material such as described in document [6]. In particular, with reference to the results in Table 1 on page 24 of this document [6], uranium(VI) loading capacities of 6.96 g and 9.72 g per kg of organic-inorganic hybrid material are respectively obtained with the same Liquid/Solid ratio, for initial synthetic aqueous phosphoric acid solutions comprising 1 mol/L of phosphoric acid and with concentrations of uranium(VI) slightly higher than those of solutions S1 and S2.

The results in Table 1 above also show that the impregnated resin allows selective extraction of uranium(VI) over iron(III). Indeed, with the above-cited Liquid/Solid ratios, loading capacities C_(U) of 8.6 g and 18.6 g of uranium(VI) per kg of impregnated resin were obtained with solutions S3 and S4 comprising both uranium(VI) and iron(III). Such values, although slightly lower than those obtained with solutions S1 and S2, remain high.

By comparison, there is a loading capacity of 7.44 g of uranium(VI) per kg of hybrid organic-inorganic material described in Table 1 of document [6] with an initial synthetic aqueous solution of phosphoric acid comprising 1 mol/L of phosphoric acid and concentrations of 3295 mg/L of iron(III) and 374 mg/L of uranium(VI). With the same Liquid/Solid ratio and a lower concentration of uranium(VI) in solution S3, the loading capacity observed with the impregnated resin remains higher.

2.2 Evaluation of U(VI) Extraction and Recovery Capacity by Column Tests

The capacity of the organic material in Example 1 to extract and then to recover the uranium(VI) contained in an aqueous phosphoric acid solution was also determined by column tests conducted following the protocol described below.

The loading capacity, denoted C_(U) and expressed in g of uranium/L of impregnated resin (denoted g U/L), was calculated with the formula given below:

$C_{U} = {\left( {C_{ini} - C_{end}} \right) \times \frac{V_{solution}}{V_{resin}}}$

where:

-   -   C_(ini): concentration of uranium(VI) in the initial synthetic         aqueous solution of phosphoric acid (in g/L)     -   C_(end): concentration of uranium(VI) in the samples collected         at the sampler outlet (in g/L)     -   V_(solution): volume of initial synthetic aqueous solution of         phosphoric acid placed in contact with the impregnated resin (in         L)     -   V_(resin): volume of packed bed of impregnated resin (in L)

The impregnated resin such as obtained in Example 1 was hydrated with demineralised water to allow the sampling of 15 mL of a packed volume of impregnated resin. These 15 mL of hydrated and impregnated resin were placed in a column having a diameter of 2 cm, the height reached by the bed of impregnated resin being close to 40 mm (ratio between bed height and column diameter in the order of 2).

2.2.1 Evaluation of Capacity to Extract U(VI) Contained in a Solution S5

An initial synthetic aqueous solution of phosphoric acid, denoted S5, having a redox potential of 550 mV (relative to the Ag/AgCl reference electrode), and comprising 1 mol/L of phosphoric acid and a concentration of uranium(VI) of 160 mg/L, was prepared and injected at 10 BV/h (BV meaning bed volume or volume of bed of impregnated resin) into the column comprising the impregnated resin.

A sampler was positioned at the outlet of the column to recover samples of the aqueous solution after contacting of the solution S5 with the impregnated resin. This aqueous solution was depleted of U(VI) since it had been extracted by the resin. The amount of uranium(VI) contained in each of the fractions of aqueous solution collected from the column was determined by ICP-AES and therefore corresponded to the amount of uranium(VI) that had not been extracted by the impregnated resin.

The amount of extracted uranium(VI) was inferred from the difference.

Appended FIG. 1 gives:

-   -   the breakthrough curve of uranium(VI), which reflects changes in         the concentration of uranium(VI) in the fractions of aqueous         solution collected at the column outlet, denoted [U]_(S5) and         expressed in mg/L, as a function of number of BVs; and     -   the curve reflecting the trend in uranium(VI) loading capacity         of the impregnated resin, denoted C_(U(S5)) and expressed in g         U(VI)/L, as a function of number of BVs.

With reference to the breakthrough curve of uranium(VI) in this FIG. 1, it is observed that for the first 50 BVs (corresponding to 0.75 L of solution S5), leakage of uranium(VI) is negligible: the U(VI) concentration in each of the sampled fractions is less than 10% of the initial concentration of uranium(VI) in solution S5. With the passing of each BV flow, uranium(VI) leakage increases and draws close to the initial concentration of uranium(VI) contained in solution S5. Indeed, the impregnated resin is then saturated with uranium(VI) and is no longer capable of extracting uranium(VI).

The impregnated resin has a uranium(VI) loading capacity of 19.2 g of uranium/L, therefore indicating that 288 mg of uranium(VI) were extracted with this impregnated resin.

2.2.2 Evaluation of the Capacity to Recover the U(VI) Extracted by the Impregnated Resin

After the extraction phase described in Chapter 2.2.1 above, the column was washed with demineralised water at 4 BV/h, to remove residual traces of phosphoric acid remaining on the impregnated resin.

The impregnated resin was then eluted with an aqueous 1.5 mol/L solution of sodium carbonate injected at 1 BV/h.

Fractions of eluate were collected at the column outlet to determine the volume thereof and the amount of U(VI) contained therein. These fractions recovered at regular intervals (every 2 h) and their characteristics are given in Table 2 below. Table 2 also gives the number of BVs of aqueous sodium carbonate solution used at these time intervals for the recovery of U(VI).

TABLE 2 Elapsed time (h) 2 4 6 8 10 12 14 Volume (ml) 24.8 26.4 27.1 25.7 25.2 25.7 35.1 Cumulative volume 24.8 51.2 78.3 104.0 129.2 154.9 190.0 (ml) BV 1.7 1.9 1.8 1.7 1.7 1.7 2.3 Cumulative BV 1.7 3.4 5.2 6.9 8.6 10.3 12.7 Quantity of eluted 173.7 94.8 8.0 3.9 2.7 2.1 2.2 U(VI), (mg) Cumulative quantity 173.7 268.5 276.5 280.4 283.1 285.2 287.4 of eluted U(VI), (mg) Cumulative elution 60.2 93.1 95.9 97.3 98.2 98.9 99.7 yield (%)

It follows from Table 2 above that the elution of uranium(VI) by the aqueous sodium carbonate solution is quantitative: more than 95% of the uranium(VI) initially extracted by the impregnated resin were eluted in 6 BVs.

After an elution time of 14 h, 99.7% of the uranium(VI) extracted by the resin had been recovered.

2.2.3 Evaluation of the Capacity to Extract U(VI) Contained in Solution S6

An initial synthetic aqueous solution of phosphoric acid, denoted S6, having a redox potential of 500 mV (relative to the Ag/AgCl reference electrode) and comprising 1 mol/L of phosphoric acid, a concentration of uranium(VI) of 160 mg/L and a concentration of iron(III) of 2.9 g/L was prepared and injected into the column comprising the impregnated resin at 10 BV/h.

As previously, a sampler was positioned at the column outlet to determine by ICP-AES analysis the amount of uranium(VI) contained in each of the fractions of aqueous solution collected from the column, and via the difference, the amount of extracted uranium(VI).

The curves reflecting the changes in concentration of uranium(VI) found in the fractions of aqueous solution collected at the column outlet (denoted [U]_(S6) and expressed in mg/L) and the trend in uranium(VI) loading capacity of the impregnated resin (denoted C_(U(S6)) and expressed in g of uranium(VI)/L) as a function of the number of BVs are given in FIG. 2, this FIG. 2 also comprising the curves of FIG. 1.

It is observed that the loading capacity C_(U) of the impregnated resin decreases slightly in the presence of iron (III) to tend towards a value of 15.5 g U(VI)/L (curve C_(U(S6))), compared with 19.2 g U(VI)/L in the presence of uranium alone (curve C_(U(S5))). The competition between the two elements U(VI) and Fe(III) is reflected by a breakthrough curve [U]_(S6) whose plateau is reached more rapidly (at about 300 BVs) compared with 370 BVs for the solution S5 (curve [U]_(S5)).

It is specified that the amount of iron(III) extracted by the impregnated resin was not determined directly via the fractions of aqueous solution collected at the outlet of the sampler; the amount of iron extracted by the resin in each of the fractions remains too slight, compared with the 2.9 g Fe/L initially contained in solution S6; the delta for iron extracted in each of the fractions cannot therefore be measured.

Failing this, mineralisation of the resin was carried out to place its iron(III) content in solution. For this mineralisation, the resin is placed in solution to assay the amounts of uranium(VI) and iron(III). To do so, the resin is first calcined and then dissolved in nitric acid.

This mineralisation allowed the determination that the amount of iron(III) present on the impregnated resin was 21 mg, a value which corresponds to an iron(III) loading capacity of the impregnated resin, denoted C_(Fe), of 1.4 g iron(III)/L.

Therefore, out of the 370 BV flows, only 0.13% of iron(III) was extracted from solution S6, which proves to be very low.

2.2.4 Evaluation of the Capacity to Recover Iron(III) Present on the Impregnated Resin, after Elution of U(VI)

After the extraction phase described in Chapter 2.2.3 above, and as previously described in Chapter 2.2.2, the resin column was washed with water at 4 BV/h, followed by elution of the uranium(VI) present on the impregnated resin with an aqueous 1.5 mol/L solution of sodium carbonate injected at 1 BV/h.

After this elution with the aqueous sodium carbonate solution, uranium(VI) was recovered quantitatively in the eluate, but not iron(III). Indeed, in the presence of sodium carbonate, iron(III) is precipitated on the resin in the form of iron hydroxide Fe(OH)₃. In addition, as seen in Chapter 2.2.3 above, by means of the selectivity of the compound meeting general formula (I), there is practically no extraction of iron (III) by the impregnated resin. The amount of iron(III) hydroxide precipitate on the resin is therefore small and does not perturb the flow of the aqueous solutions of sodium carbonate or of sulfuric acid in the resin column.

After eluting the U(VI), the column was washed a second time with demineralised water at 4 BV/h, followed by elution of the resin column using an aqueous 3.5 mol/L solution of sulfuric acid at 1 BV/h to recover iron(III).

This elution allows the impregnated resin to be depleted of the iron present in the form of Fe(OH)₃ and allows regeneration of the DEHCNPB extractant of the impregnated resin. The phosphonic acid function of the extractant, that was deprotonated at the elution step with the aqueous sodium carbonate solution, is reprotonated when eluting with the aqueous sulfuric acid solution.

The impregnated resin is then ready for a new extraction/stripping cycle.

To support of these facts, monitoring of iron(III) was carried out.

Fractions of aqueous solution were collected at the column outlet at regular intervals (every 2 h) when eluting with this aqueous solution of sulfuric acid, and then analysed.

The characteristics of these fractions (volume or amount of eluted iron(III)) are given in Table 3 below. This Table 3 also indicates the number of BVs used over these time intervals.

It is recalled that, when eluting uranium(VI) with the sodium carbonate solution, only 1.5 mg of iron(III) was previously eluted.

TABLE 3 Elapsed time (h) 2 4 6 8 10 12 14 Volume (ml) 26.5 28.8 29.8 28.0 28.2 27.8 20.2 Cumulative volume 26.5 55.3 85.0 113.0 141.2 169 189.2 (ml) BV 1.8 1.9 2.0 1.9 1.9 1.8 1.3 Cumulative BV 1.8 3.7 5.7 7.5 9.4 11.3 12.6 Quantity. of collected 4.1 6.2 4.4 1.8 0.9 0.5 0.0 Fe(III) (mg) Cumulative quantity. 4.1 10.3 14.7 16.5 17.4 17.9 17.9 of collected Fe(III) (mg) Consumed H⁺ (eq/L) 3.8 6.7 7.0 7.3 7.3 7.2 7.3 Concentration of 1.9 3.3 3.5 3.6 3.6 3.6 3.6 H₂SO₄ (mol/L)

After elution with the 3.5 mol/L aqueous sulfuric acid solution, the impregnated resin was mineralised after which it was shown that this resin only contained 112 mg of Fe(III)/L. This iron(III) loading capacity of the impregnated resin, denoted C_(Fe), is particularly low since it only represents 1.6 mg Fe, i.e. 7% of the complexed iron initially on the resin.

This elution with 3.5 mol/L aqueous solution of sulfuric acid allowed good removal of iron (yield of 85%) and regeneration of the impregnated resin. This regeneration can be demonstrated by assay, using sodium hydroxide, of the acidity of each of the fractions of aqueous acid solution collected after elution. The results obtained are also given in Table 3 above.

It is observed that for the first two collected fractions, 45% and 5% of the sulfuric acid of the initial 3.5 mol/L aqueous solution of sulfuric acid used for this elution were indeed consumed to regenerate the resin extractant and simultaneously carry out iron removal.

2.2.5 Comparison Between Capacities to Extract U(VI) Contained in a Solution S7 and to Recover this U(VI) and Fe(III) Extracted by the Resin

An initial synthetic aqueous solution of phosphoric acid, denoted S7, comprising 1 mol/L of phosphoric acid, a concentration of uranium(VI) of 151 mg/L and a concentration of iron(III) of 2.9 g/L was prepared and injected, at 10 BV/h, into three columns of the type described above in Chapter 2.2 but respectively comprising:

-   -   15 mL of impregnated resin, or organic material, conforming to         the invention, denoted Ri,     -   15 mL of a resin, denoted R1, formed by a styrene/divinylbenzene         copolymer and phosphonic groups in sodium form, marketed under         the trade name Lewatit® TP 260; and     -   15 mL of a resin, denoted R2, formed by a crosslinked         polystyrene and di-(2-ethylhexyl) phosphoric acid groups         (D₂EHPA), marketed under the trade name Lewatit® VP OC 1026.

It is specified that these resins R1 and R2, both marketed by the Lanxess Company, are known for the extraction and recovery of uranium(VI) contained in aqueous phosphoric acid solutions. In particular, resin R1 is one of the resins potentially used in document [5].

With reference to FIG. 3 illustrating the trend in uranium(VI) loading capacity as a function of number of BVs, it is observed that, right from the first BVs, leakage of uranium(VI) is observed with the commercial resins R1 and R2, contrary to the impregnated resin Ri which has much higher uranium(VI) loading capacity.

For each of the solid polymeric substrates, the U(VI) loading capacity was determined by measuring the uranium concentrations in all the fractions collected at the column outlet. After extraction and stripping of uranium(VI), each of these resins was mineralised to determine the amounts of iron(III) extracted by each of these resins Ri, R1 and R2.

The corresponding results, and the calculated values allowing determination of the selectivity of extraction of uranium(VI) over iron(III), are given in Table 4 below.

TABLE 4 U(VI) loading Fe(III) loading Fe(III)/U(VI) ratio Resin capacity (g U(VI)/L) capacity (g Fe(III)/L) (%) Ri 15.5 1.4 9.0 R1 2.7 24.5 918 R2 1.7 3.3 191

It follows from the data in Table 4 above that the impregnated resin Ri used in the method of the invention allows values to be reached for uranium(VI) loading capacity, and hence for extraction, that are distinctly higher than those obtained with the commercial resins R1 and R2 commonly used to extract uranium (VI) from aqueous phosphoric acid solutions. In addition, the impregnated resin Ri also allows the extraction of this uranium(VI) with selectivity over iron(III) that is undeniably greater than that imparted by these resins R1 and R2.

Example 3: Properties of the Material of the Invention for the Extraction and Recovery of Uranium(VI) from Aqueous Phosphoric Acid Solutions

The three resins Ri, R1 and R2 used in Chapter 2.2.5 above were evaluated for their performance in extracting uranium(VI) contained in an aqueous phosphoric acid solution formed no longer by a synthetic solution but by an industrial solution.

This industrial solution, denoted SI, initially comprised 5 mol/L of phosphoric acid, 130 mg/L of uranium(VI) and 2.8 g/L of iron(III) with a redox potential of 490 mV (relative to the Ag/AgCl reference electrode).

As previously, 15 mL of each of the resins Ri, R1 and R2 were placed in a column having a diameter of 2 cm until a bed height of 40 mm was reached, to obtain a ratio of 2 between bed height and column diameter.

The industrial solution SI was injected into each of the columns at a rate of 10 BV/h for a time of 26 h.

The breakthrough curves, reflecting the changes in the concentration of uranium(VI) found in the fractions of aqueous solution collected at the column outlet, as a function of number of BVs, are given in FIG. 4 for each of the resins Ri, R1 and R2.

This FIG. 4 shows that the leakage of uranium(VI) is limited during the 20 first BVs when using the impregnated resin Ri.

With reference to FIG. 5 illustrating the trend in uranium(VI) loading capacity as a function of number of BVs, it is observed that, right from the first BVs, a plateau is reached for the loading capacities of the commercial resins R1 et R2, proving that these two resins are very rapidly saturated; as a result, they have relatively small uranium(VI) loading capacities. On the contrary, the loading capacity of the impregnated resin Ri increases progressively with each BV flow to reach a uranium(VI) loading capacity that is much better: 7 g U(VI)/L.

The decrease in loading capacity of uranium(VI) that is here observed compared with that of 15.5 g U(VI)/L measured in Chapter 2.2.3 (curve C_(U(S6))) can be explained by the concentration of phosphoric acid in the aqueous solution SI from which it is desired to extract uranium(VI). Indeed, at a concentration of 5 mol/L of phosphoric acid, and no longer of only 1 mol/L, uranium(VI) is more strongly complexed in this aqueous solution S1 and is therefore more difficult to extract.

For each of the solid polymeric substrates, the U(VI) loading capacity was determined by measuring the uranium(VI) concentrations in all the fractions collected at the outlet of the column. After extraction followed by stripping of uranium(VI), the organic materials were mineralised on each of the three columns to determine the amounts of iron(III) extracted by each of the resins Ri, R1 and R2.

The corresponding results, and the calculated values allowing determination of the selectivity of extraction of uranium(VI) over iron(III), are given in Table 5 below.

TABLE 5 U(VI) loading Fe(III) loading Fe(III)/U(VI) ratio Resin capacity (g U(VI)/L) capacity (g Fe(III)/L) (%) Ri 6.9 0.74 11 R1 1.0 16.2 1620 R2 0.1 0.46 460

Table 5 confirms the decrease in loading capacity of uranium(VI) but also of iron (III) with the increased molar concentration of phosphoric acid in the aqueous solution from which it is desired to extract uranium.

However, it is undeniable that the impregnated resin used in the extraction and recovery methods conforming to the invention by far is the best compromise for the extraction of uranium(VI), and in a manner that is particularly selective with regard to iron(III).

BIBLIOGRAPHY

[1] U.S. Pat. No. 3,711,591

[2] WO 2013/167516 A1

[3] U.S. Pat. No. 4,599,221 [4] U.S. Pat. No. 4,402,917

[5] WO 2014/018422 A1 [6] WO 2014/127860 A1 

What is claimed is:
 1. Method for extracting uranium(VI) from an aqueous solution S comprising phosphoric acid, this method including placing the aqueous solution S in contact with an organic material, followed by separation of the aqueous solution and the organic material, characterized in that the organic material comprises a solid polymeric substrate impregnated with a compound meeting following general formula:

where: m is an integer of 0, 1 or 2; R¹ and R², the same or different, are a linear or branched, saturated or unsaturated hydrocarbon group having 6 to 12 carbon atoms; R³ is: a hydrogen atom; a linear or branched, saturated or unsaturated hydrocarbon group having 1 to 12 carbon atoms and optionally one or more heteroatoms; a saturated or unsaturated hydrocarbon group comprising one or more rings of 3 to 8 carbon atoms, the ring(s) optionally comprising one or more heteroatoms; or an aryl group comprising one or more rings, the ring(s) optionally comprising one or more heteroatoms; or else R² and R³ together form a group —(CH₂)_(n)— where n is an integer ranging from 1 to 4; R⁴ is: a linear or branched, saturated or unsaturated hydrocarbon group having 2 to 8 carbon atoms; a saturated or unsaturated hydrocarbon group comprising one or more rings, the ring(s) optionally comprising one or more heteroatoms; or an aromatic group comprising one or more rings, the ring(s) optionally comprising one or more heteroatoms; and R⁵ is a hydrogen atom or a linear or branched, saturated or unsaturated hydrocarbon group having 1 to 12 carbon atoms.
 2. The extraction method according to claim 1, wherein the compound meets the following particular formula (I-a):

where: R¹ and R² are an alkyl group having 8 to 10 carbon atoms; one from among R³ and R⁵ is a hydrogen atom and the other from among R³ and R⁵ is an alkyl group having 4 to 10 carbon atoms; and R⁴ is an alkyl group having 4 to 6 carbon atoms.
 3. The extraction method according to claim 1, wherein the compound is selected from among: ethyl 1-(N,N-diethylhexylcarbamoyl)ethylphosphonate, meeting the particular formula (I-a) where R¹ and R² are both a 2-ethylhexyl group, R⁴ is an ethyl group, one from among R³ and R⁵ is a hydrogen atom whilst the other from among R³ and R⁵ is a methyl group; ethyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, meeting the particular formula (I-a) where R¹ and R² are both a 2-ethylhexyl group, R⁴ is an ethyl group, one from among R³ and R⁵ is a hydrogen atom whilst the other from among R³ and R⁵ is an n-octyl group; butyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, meeting the particular formula (I-a) where R¹ and R² are both a 2-ethylhexyl group, R⁴ is an n-butyl group, one from among R³ and R⁵ is a hydrogen atom whilst the other from among R³ and R⁵ is an n-octyl group; butyl 1-(N,N-dioctylcarbamoyl)nonylphosphonate, meeting the particular formula (I-a) where R¹ and R² are both an n-octyl group, R⁴ is an n-butyl group, one from among R³ and R⁵ is a hydrogen atom whilst the other from among R³ and R⁵ is an n-octyl group; and isopropyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate, denoted DEHCNPIP, meeting the particular formula (I-a) above where R¹ and R² are both a 2-ethylhexyl group, R⁴ is an isopropyl group, one from among R³ and R⁵ is a hydrogen atom whilst the other from among R³ and R⁵ is an n-octyl group, the compound preferably being butyl 1-(N,N-diethylhexylcarbamoyl)nonylphosphonate.
 4. The extraction method according to claim 1, wherein the solid polymeric substrate is formed of a polymer comprising at least one repeat unit selected from among an olefin unit, benzene unit, acrylic ester unit and mixtures of these units, this polymer advantageously being a divinylbenzene/styrene copolymer or an acrylic ester polymer.
 5. The extraction method according to claim 1, wherein the solid polymeric substrate has a specific surface area, determined by the BET method, of between 300 m²/g and 1000 m²/g.
 6. The extraction method according to claim 1, wherein the solid polymeric substrate is in the form of beads of which at least 90% by number advantageously have a mean bead size d₉₀ of between 200 μm and 900 μm.
 7. The extraction method according to claim 1, wherein the organic material comprises at least 2.5 wt. %, advantageously from 10 wt. % to 70 wt. % and, preferably, from 20 wt. % to 60 wt. % of compound.
 8. The extraction method according to claim 1, wherein the contacting of the aqueous solution S with the solid organic material comprises at least one circulation of a mobile phase formed by the aqueous solution S, on a stationary phase formed by the organic material.
 9. Method for recovering uranium(VI) from an aqueous solution S comprising phosphoric acid, said method comprises: (a) extracting uranium(VI) from the aqueous solution S with an extraction method according to claim 1; and (b) placing the organic material obtained after step (a) in contact with a basic aqueous solution, followed by separation of the organic material and the basic aqueous solution after which uranium(VI) is recovered in the basic aqueous solution.
 10. The recovery method according to claim 9, wherein the placing in contact of the organic material obtained after step (a) with the basic aqueous solution comprises the eluting, with a mobile phase formed by the basic aqueous solution, of a stationary phase formed by the organic material.
 11. The recovery method according to claim 9, wherein the basic aqueous solution has a pH of 8 or higher, advantageously of between 8.5 and 12 and, preferably, of between 9 and
 11. 12. The recovery method according to claim 9, wherein the basic aqueous solution is an aqueous solution of an alkaline metal salt or ammonium salt, the salt being advantageously selected from among a carbonate and the metal being advantageously selected from among sodium and potassium.
 13. The recovery method according to claim 9, further comprising regeneration of the organic material, the regeneration comprising, after step (b): (c) placing the organic material obtained after step (b) in contact with an acid aqueous solution, followed by separation of the organic material and the acid aqueous solution after which the organic material is regenerated.
 14. The recovery method according to claim 14, further comprising, between steps (b) and (c): (b′) washing with water of the organic material separated at step (b).
 15. The recovery method according to claim 13, wherein the contacting of the organic material separated at step (b), and optionally washed at step (b′), with the acid aqueous solution comprises at least one washing with a mobile phase formed by the acid aqueous solution, on a stationary phase formed by the organic material.
 16. The recovery method according to claim 13, wherein the aqueous acid solution has a concentration of if ions of 20 mol/L or less, advantageously between 0.1 mol/L and 10 mol/L and, preferably, between 1 mol/L and 7 mol/L.
 17. The recovery method according to claim 13, wherein the aqueous acid solution comprises at least one inorganic acid selected from the group formed by sulfuric acid, nitric acid, phosphoric acid and hydrochloric acid, this inorganic acid being advantageously sulfuric acid.
 18. The extraction method according to claim 1, wherein the aqueous solution S comprises at least 0.1 mol/L, advantageously from 1 mol/L to 10 mol/L, preferably from 2 mol/L to 9 mol/L and, more preferably, from 3 mol/L to 7 mol/L of phosphoric acid.
 19. The extraction or recovery method according to claim 18, characterized in that the aqueous solution S is a solution resulting from attack of a natural phosphate by sulfuric acid.
 20. The recovery method according to claim 9, wherein the aqueous solution S comprises at least 0.1 mol/L, advantageously from 1 mol/L to 10 mol/L, preferably from 2 mol/L to 9 mol/L and, more preferably, from 3 mol/L to 7 mol/L of phosphoric acid.
 21. The extraction or recovery method according to claim 20, characterized in that the aqueous solution S is a solution resulting from attack of a natural phosphate by sulfuric acid. 